ACME协议可以用于获取新证书以及更新已有证书。下面是一个使用ACME协议更新SSL证书的示例代码：

import requests
import json

# 获取新的ACME V2证书
def get_new_cert(domain, email):
    # 根据需要更改以下变量
    acme_directory_url = 'https://acme-v02.api.letsencrypt.org/directory'
    account_key_file = 'path/to/account/key.pem'
    domain_key_file = 'path/to/domain/key.pem'

    # 获取目录URL
    r = requests.get(acme_directory_url)
    directory = json.loads(r.content)

    # 获取带有新证书请求的Payload
    new_cert_url = directory['newCert']
    headers = {'Content-Type': 'application/jose+json'}
    payload = {
        'protected': {
            'alg': 'RS256',
            'jwk': # 引用账户公钥的内容，
            'nonce': r.headers['Replay-Nonce']
        },
        'payload': # 直接引用CSR（包含公钥)
        'signature': # 使用RSA签名私钥进行签名
    }
    # 向ACME服务器发送新证书请求
    r = requests.post(new_cert_url, headers=headers, data=json.dumps(payload))

    # 保存证书和密钥
    cert = r.content
    with open(domain_key_file, 'w') as f:
        f.write(cert)

# 更新ACME V2证书
def update_cert(cert_url, account_key_file, domain_key_file):
    # 获取目录URL
    r = requests.get(cert_url)
    cert_info = json.loads(r.content)
    domain = cert_info['identifier']['value']

    # 获取证书请求的Payload
    headers = {'Content-Type': 'application/jose+json'}
    payload = {
        'protected': {
            'alg': 'RS256',
            'kid': # ACME账户的URL,
            'nonce': r.headers['Replay-Nonce